The present invention relates in general to emergency call processing and more particularly to a system for special call handling and providing identification information from a multi-line telephone system to a public safety answering point (PSAP) for a call requiring specific location identification.
An important requirement for emergency 911 call systems is the identification of the location of a caller initiating an emergency call. For example, in situations where a caller is unable to communicate with the emergency operator, the automatic identification of the location of the call enables the operator to dispatch emergency services to the physical location from which the call originated.
The identification of the caller initiating an emergency call is rendered difficult in multi-line systems such as PBXs because a plurality of extensions, some of which may be at disparate locations, are all identified by the same pilot number. Thus, it is difficult to pinpoint the exact location of the originating call from among the plurality of extensions.
U.S. Pat. No. 5,347,568 (Moody, et al) discloses a solution to the problem of positively identifying a caller from among a plurality of extensions in a multi-line telephone system. Specifically, a dedicated adjunct (referred to in the ""568 Patent as a station translation system (STS)) is connected to the PBX via a dedicated tie line. In response to a user initiating a 911 emergency call, the call is recognized by the PBX as being an emergency call and is routed to the STS, rather than via the normal PBX trunk lines to the central office (CO), which then identifies the calling party station, harmonizes the protocol of the PBX to an ANI (Automatic Number Identification) number which is recognizable by the emergency call network, and routes the call to dedicated trunks in the emergency call network.
Although the system disclosed in the ""568 Patent addresses the problem of identifying the location of a station originating an emergency call in a multi-line system, an expensive and difficult interface is required between the PBX and STS, which permanently occupies a slot in the PBX and requires adjunct hardware and special emergency trunks to connect to the emergency network.
Considerable discussion and research has been engendered concerning the desirability of using ISDN (Integrated Services Digital Network) for providing 911 emergency services. Unfortunately, standards committees have thus far been unable to propose an acceptable solution to the problem of how to handle location identification in the ISDN protocol and have not addressed other special needs of an emergency call such as the requirement for guaranteed channel availability. In particular, existing COs typically screen any calling number IDs and other information assigned to a call by a user. Also, with call-by-call service it is possible that all channels may be busy when an emergency call is required, such that the call becomes blocked within the PBX.
According to the present invention, a system is provided for assigning a number which is designed to pass the screening test imposed by the COs, for identifying a physical location within a geographical area serviced by a PBX. A table is provided in the PBX (or a dedicated server connected thereto), for assigning a number associated with the telephone""s physical location. The problem of emergency channel reservation is overcome according to the present invention by providing a channel allocation algorithm that always keeps one or more ISDN channels open for handling emergency calls.